S22A0094

Understanding the evolution of the supermassive black hole at z>6–7 is one of the long-standing challenges in astronomy. A classical scenario predicts an evolutionary sequence of a dusty red quasar emerging from a heavily dust-obscured starburst phase, and transitioning to a luminous blue quasar. The latter has been identified up to z=7.6, while no red quasars have been found at z>7 until now. Here we propose Keck/MOSFIRE H, K-band spectroscopy for the least luminous, the most distant red quasar discovered at z=7.2 by our full HST archive survey. NOEMA DDT observations securely determine the redshift with the [C ii]158-µm line at z=7.1899, and full SED analysis reveals a vigorous star-formation rate of ∼1,600 M_⊙/yr (×∼3-15 higher than other z>7 quasars hosts). An extremely faint X-ray property is also identified, suggesting that a less massive black hole (<10⁸M_⊙) is rapidly growing with super-Eddington accretion. To verify the rapidly growing nature, we aim at evaluating i) the black-hole mass and evaluating the Eddington ratio via Mgii, and ii) the nuclear outflow velocity via Ciii]. This source is the first candidate of a rapidly growing BH in a dusty starburst at z>7 and thus represents a unique opportunity to solve this long-standing challenge in astronomy.

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